BAGHDAD, Iraq — Iraq’s embattled prime minister vowed Thursday to pursue his bid for a second term despite pressure from home and abroad to step down, signaling no early end to the standoff blocking a crucial national unity government.

Shiite politicians suggested they may turn to Grand Ayatollah Ali al-Sistani, the sole figure with the authority to make a decision that risks shattering Shiite unity.

In a brutal reminder of the stakes if Iraqi leaders cannot reverse the slide toward chaos, a car bomb exploded Thursday in the country’s most sacred Shiite city, Najaf, killing 10 people and wounding more than 30.

Prime Minister Ibrahim al-Jaafari told reporters he would relinquish his mandate only if parliament refuses to approve him or if the seven groups within the Shiite alliance withdraw their nomination, which he won by a single vote in a caucus in February.

The Shiite bloc controls 130 of the 275 parliament seats, enough for first crack at the prime minister’s job but not enough to govern without Sunni and Kurdish partners. But the Sunnis and Kurds demand that al-Jaafari be replaced, blaming him for the sharp rise in sectarian tensions that threatens to plunge the country into civil war.

Al-Jaafari has refused to stand down despite pressure from Secretary of State Condoleezza Rice and British Foreign Secretary Jack Straw, who personally urged the Iraqis to break the logjam during a two-day visit this week.

Political showdown could prompt violenceShiite officials fear a showdown over al-Jaafari could tear apart the Shiite alliance and risk a violent reaction from radical cleric Muqtada al-Sadr, who runs the feared Mahdi Army militia and is a key supporter of the prime minister.

To break the deadlock, Sunni and Kurdish politicians suggested that parliament convene Wednesday to decide al-Jaafari’s fate. But Shiite officials decided Thursday to delay the session until all Iraqi parties agree on nominees for other posts, including the national president and speaker of parliament, Shiite politician Khalid al-Attiyah said.

Al-Attiyah said the impasse had become “very complicated” and al-Jaafari’s supporters within the alliance want to ask the advice of al-Sistani, the country’s most respected Shiite cleric.

That would give Shiite politicians political cover and could avoid a showdown with al-Sadr.

It is uncertain, however, whether al-Sistani wants to become involved in an internal Shiite political struggle. Unlike his counterparts in Iran, he has long maintained that clerics should remain above politics and instead offer moral guidance.

Al-Sistani’s aides have said the Iranian-born cleric has become frustrated with the performance of Shiite religious parties, which dominate the outgoing government, and with the rising tensions between Shiites and Sunnis.

But the weakness of Iraqi political institutions, which were revived only after the 2003 collapse of Saddam Hussein’s regime, has prompted al-Sistani to take stands on political issues, especially during the early months of the U.S. occupation.

Al-Sistani’s repeated demands for elections forced several changes in the U.S. blueprint for restoring Iraqi sovereignty and prompted the Americans to speed up their timetable for the first nationwide ballot in January 2005.

Turning to al-Sistani, however, would be a tacit acknowledgment by Shiite political leaders that they lack both the stature and the political legitimacy to make difficult and potentially divisive decisions.

Respect from AmericaThe Americans have long acknowledged his pre-eminent leadership role within the Shiite community, which accounts for about 60 percent of Iraq’s 27 million people. Last month President Bush sent a letter to al-Sistani thanking him for appealing for calm.

U.S. Ambassador Zalmay Khalilzad has met with most top Iraqi politicians about forming a new government, but the ayatollah has steadfastly refused to meet with any American official.

During her visit to Baghdad, Rice praised al-Sistani for helping to curb Shiite reprisals against Sunni extremists responsible for car bombs and suicide attacks that have killed hundreds of Shiite civilians.

The latest attack occurred Thursday afternoon in Najaf, where al-Sistani lives in virtual seclusion 100 miles south of Baghdad. Police and witnesses said the blast took place about 330 yards from the Imam Ali mosque, the tomb of the Prophet Muhammad’s son-in-law and one of the most sacred shrines for Shiites.

Police sealed off much of central Najaf and ordered people to leave for fear other bombs may be hidden there. Such attacks are rare in Najaf, which is tightly controlled by police and Shiite security guards, and are seen by Shiites as a grave provocation because of the city’s stature.

In a statement, State Department spokesman Sean McCormack condemned the bombing but asked “all Iraqis to exercise restraint in the wake of this tragedy, and to pursue justice in accordance with the laws and constitution of Iraq.”

The bombing of the golden dome of a Shiite shrine in Samarra on Feb. 22 triggered a deadly wave of reprisal attacks against Sunni mosques and clerics.

More violence across the nationAt least 11 other people were killed Thursday across Iraq, including four Iraqi police and soldiers. The civilian deaths included five Shiite truck drivers ambushed south of the capital and two people shot dead in Baghdad, police said.

In addition, the bodies of five men — four in Baghdad and one in Kirkuk — were found Thursday, apparent victims of sectarian killings, police said. It was uncertain when they died.

Two Sunni Arab politicians — Khalaf al-Ilyan, head of the National Dialogue Council, and Saleh al-Mutlaq — said Thursday that close relatives had disappeared.

“Al-Qaida in Iraq is behind this to put pressures on us to quit the political process as they previously threatened us not to take part in it,” al-Ilyan told The Associated Press.

Meanwhile, the U.S. military announced the arrest of an insurgent leader believed responsible for many of the attacks against Shiites and for the February 2005 kidnapping of Italian journalist Guiliana Sgrena.

Mohammed Hila Hammad Obeidi, also known as Abu Ayman, was arrested last month south of Baghdad, but the announcement was delayed until DNA tests confirmed his identity, the U.S. statement said.

Obeidi, a former member of Saddam’s intelligence service, allegedly commanded the Secret Islamic Army in Babil province south of Baghdad and is believed to have ties to the leader of al-Qaida in Iraq, Abu Musab al-Zarqawi.

Sgrena was freed after a month’s captivity. The Italian agent who secured her release was killed by U.S. gunfire as they headed to Baghdad airport on March 4, 2005.